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[dsp] AFSK demodulator code for ARM7TDMI?

Scott Miller scott at opentrac.org
Fri Feb 11 22:16:43 UTC 2005

> If you purchase the ARM developers guide there are examples of FIR filters
> and a radix-4 FFT implementation. The speed is adequate for any
> processing data from an audio CODEC.

Thanks, I'll have to look into that.  That brings up another question...
what's best for decoding AFSK - Goertzel algorithm, FFT, quadrature
demodulator, etc?  From the description I read, the Goertzel algorithm
sounded less CPU-intensive than a regular FFT for a small number of
frequencies, but I think it might take too long a sample period to be very
useful.  The quadrature demodulator I'm still learning about - seems like it
needs a certain amount of low-pass filtering.  I understand some of the
basic concepts, but the reality of their implementation is another matter.

> Since I couldn't find a fast real-time OS for the ARM7, I ended up writing
> one. If anyone wants a copy for amateur radio applications, send me an
> email.

Define 'fast'.  I'm using FreeRTOS.  Works so far, but then I'm not doing
any signal processing yet!  I'm hoping that the peripheral data controller
functions will help keep my interrupt processing overhead low - if I can
load sizeable blocks of data from the codec without an interrupt each byte,
it'll mean a lot less interrupts.


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